The present invention relates to a system for generating stitch patterns for a sewing machine in which an information carrying member with multiple pieces of pattern information recorded thereon to control a stitch forming instrumentality is rotated by a step motor in forward and reverse directions, and a scanning member performs selective pickup of the pattern information sequentially with rotation of the information carrying member, thereby controlling the stitch forming instrumentality according to the pattern information thus picked up.
There is known a prior system for controlling the position of lateral oscillation of a needle in accordance with rotation of a bight information carrying member caused by a step motor. In such a system, a contact element operatively connected to a needle bar gate is engageable with an eccentric cam secured to the output shaft of the step motor, and the position of lateral oscillation of the needle is determined depending on the level of a cam surface with which the contact element comes into engagement. Accordingly, for enabling the needle to move from one extreme position to the other in its lateral oscillation by a single step synchronously with rotation of the main shaft of the sewing machine, it becomes necessary for the step motor to rotate the eccentric cam through a maximum angle. And this signifies that a remarkably high response speed is required of the step motor. That is, if the step motor fails in satisfying the requirement of such response performance, setting of the eccentric cam at a desired angular position during a high-speed operation mode of the sewing machine cannot be executed in completely timed relation to the rotation of the main shaft, so that there occurs an error in the needle position for penetration through work fabric as a result to cause disorder in the stitch pattern being formed.
Since the response performance of the step motor depends on the structure of the motor itself and a drive control means employed therefor, a great increase of the cost is unavoidable to achieve enhanced performance. Thus, it has been extremely difficult heretofore to accomplish a low-cost sewing machine of the type that controls the position of lateral oscillation of the needle by means of a step motor for generating a variety of stitch patterns.